Chemical Geology最新文献

Volcanic clasts from the International Ocean Discovery Program Site U1431 in the South China Sea (SCS) were analyzed for V isotopic compositions in this study. These volcanic clasts represent mantle-derived carbonated melts and record the transition from carbonated melts to alkali basalts. Based on the formation sequence, these clast samples were divided into early-stage (> 8.3 Ma) and late-stage (< 8.3 Ma) clasts. The early-stage volcanic clasts have δ⁵¹V values of −0.76‰ to −0.67‰, which provide an estimate for the V isotopic composition of primary carbonated melts. Their V isotopic compositions are slightly heavier than those of mid-ocean ridge basalts (−0.84‰ ± 0.10‰) and bulk silicate Earth (−0.86‰ to −0.91‰), reflecting the control of low-degree partial melting under conditions of high oxygen fugacity. The late-stage volcanic clasts have δ⁵¹V values ranging from −0.62‰ to 0.29‰, which are systematically heavier than those of early-stage volcanic clasts. The carbonated melts rose through and reacted with the lithospheric mantle and were transformed into alkali basalts by dissolving orthopyroxene and precipitating olivine and/or clinopyroxene. Because the reactants and reaction products do not lead to significant V isotope fractionation, the V isotopes show limited variations during transformation from carbonated melts to alkali basalts. The alkali basalts were emplaced into shallow crust by multiple pulses, in which the magma experienced variable degrees of magma differentiation, with δ⁵¹V values close to those of the initial alkali basalts or increasing due to fractional crystallization of FeTi oxides. The change in V isotopic compositions from early-stage volcanic clasts to late-stage volcanic clasts was driven by the varying magnitude of influence exerted by different magmatic processes en route from the deep mantle to the shallow crust.

Understanding the phase equilibria and physical-chemical characteristics of the CH₄–CO₂–H₂O–NaCl quaternary system is important for evaluating costs and risks for the storage of CO₂ in depleted natural gas reservoirs as well as fluid inclusion studies. In this study, phase equilibria and thermodynamic properties of this system were investigated through the utilization of a statistical association fluid theory-based (SAFT) equation of state (EOS) at temperatures from 298 to 513 K (25–240 °C), pressures up to 600 bar (60 MPa) and concentration of NaCl up to 6 mol/kgH₂O. The model parameters were obtained from the fitting of available experimental data of subsystems (i.e., CH₄–H₂O, CH₄–CO₂, and CH₄–H₂O–NaCl) that were judged to be reliable and incorporation of available parameters for the subsystems (i.e., pure component, CO₂–H₂O, and CO₂–H₂O–NaCl). Using the SAFT EOS developed in this study, we predicted the solubility of (CH₄ + CO₂) gas mixtures in pure H₂O and compared it with the available experimental data and the predicted values from four popular numerical simulators. The results indicate that our model can provide reliable predictions for the CH₄–CO₂–H₂O ternary system. Subsequently, we further predicted the phase equilibria and density of the CH₄–CO₂–H₂O–NaCl system with NaCl varying from 0 to 6 mol/kgH₂O. We also employed the SAFT EOS to predict the solubility of CO₂ and CH₄ in the water-alternating-gas process for CO₂-enhanced oil recovery, demonstrating good agreement with the simulation results obtained through the Peng-Robinson EOS for predicting the CO₂ and CH₄ solubility. These predicted thermodynamic properties and phase behaviors in the CH₄–CO₂–H₂O–NaCl system provide quantitative insights into the implications of CO₂ storage in depleted oil and gas reservoirs.

The lack of strong geothermal manifestations and limited fumarolic activity has sparked widespread debate about the presence of active magma reservoirs beneath the Wudalianchi volcanic field (WVF). Helium (He) in hydrological systems is sensitive and responsive to the mixture of mantle-derived He, making it an excellent tool for deciphering mantle-derived magmatic processes. Through investigating dissolved He and inorganic carbon (DIC) in shallow groundwater of the WVF, we found that He captures mantle characteristics with R/Ra ratios (R equals to ³He/⁴He ratios, Ra is atmospheric ³He/⁴He ratio of 1.39 × 10⁻⁶) ranging from 0.07 to 2.27 and the highest proportion of mantle-derived He is 30.38 %. In contrast, DIC shows minimal mantle influence, with low concentrations (1.91–7.39 mmol/L) and depleted δ¹³C-DIC values ranging from −17.9 ‰ to −13.6 ‰. The calculated mantle ⁴He fluxes in groundwater are 2–3 orders of magnitude higher than that in stable continents and show the signature of volcanic degassing. The high mantle He fluxes and high upward flow rates suggest the efficient release and transfer of mantle-derived He into shallow groundwater through magma degassing. Magma-aging model effectively constraints the period of magmatic activity to within the last 47 ka. These findings, consistent with seismic noise imaging results, support the presence of magma reservoirs in the upper–middle crust. The absence of magmatic carbon signature in groundwater suggests the decoupling of magmatic He and magmatic CO₂. Magmatic CO₂ may be removed during ascent, possibly being trapped through dissolution or precipitation processes within the low-resistivity bodies identified at ∼2.5 km depth by magnetotelluric imaging. These findings not only contribute to resolving the controversy surrounding the existence of magma reservoirs beneath the WVF but also have broader implications in guiding the exploration and validation of hidden magmatic systems.

Apatite serves as a notable host for diverse volatiles, but experimental data concerning the mobilities of volatiles out of its lattice remains scarce. Despite several investigations elucidating the diffusivities of F, Cl, and OH, their interplay mechanism remains poorly understood. Herein, to unveil the mobilities of hydrogen, fluorine, and carbon out of the apatite lattice and their intricate interaction mechanisms during high temperature processes, we carry out annealing experiments at 900, 1000, and 1100 °C on two apatite samples with distinct fluorine contents. Our findings indicate that along the c-axis, hydrogen diffusion coefficients surpass those of fluorine by 2–3 orders of magnitude, with lower activation energies observed. Hydrogen diffusion shows strong anisotropy, with lager diffusivities and lower activation energies along the c-axis than the a-axis. For both hydrogen and fluorine, the Durango apatite shows smaller diffusivities along the c-axis than the Imilchil apatite. Contrarily, carbon diffusion is absent in both samples at these temperatures. These differences in diffusivity may suggest that hydrogen diffuses as OH⁻ along the a-axis and as H⁺ along the c-axis, while fluorine diffusion relies on hydrogen diffusion along the c-axis. Based on these new data, the capacity of apatite to record volatiles can be evaluated. All volatiles, including hydrogen, is likely preserved within the core of apatite during thermal events from several days to years, such as rapid volcanic eruptions. However, for prolonged thermal events lasting hundreds of millions of years, apatite preserves volatiles for F, Cl and C, but not hydrogen.

The Baja California peninsula forms the western margin of the Gulf of California (GC) rift system, which is an active tectonic setting manifested by seismicity and numerous geothermal sites. The present study examines the geochemistry of thermal fluids (major ions and gas species, δ¹⁸O-δD, δ¹³C, ³He/⁴He) from the southern tip of the peninsula (Los Cabos Block, LCB). Sampling was mostly focused on the coastal thermal manifestations in the towns of Buenavista and El Sargento, but other sites further inland are also included for broadening the scope of the study. The main objectives include: (i) characterize water-rock interactions and other processes controlling the fluid composition, (ii) constrain solute geothermometry estimates through multi-step geochemical modeling, and (iii) discuss the fluid origins in terms of tectonics and regional shear-wave velocity anomalies in the upper lithosphere. The geothermal systems in the area have a tectonic origin and result from fluid circulation along regional faults that penetrate the upper crust through the granitic basement. Mixing between the thermal fluids and seawater at the coast is clearly illustrated through major ions and δ¹⁸O-δD relationships. Reconstruction of the pre-mixing chemical compositions indicates a low-salinity fluid at Buenavista (Cl = 104–109 mg L⁻¹) and a saline fluid at El Sargento (Cl = 7169 mg L⁻¹). The geochemical modeling allows us to validate these endmember compositions and to address a common issue when using solute geothermometry, which is the uncertainty on the state of equilibrium of the thermal fluid with respect to wall-rock minerals. The corresponding results reveal contrasting thermal regimes at depths (Buenavista: 101–122 °C, El Sargento: 212–220 °C), likely associated with differences in geothermal gradient and fluid circulation depth. Our gas samples have among the lowest He isotopic ratios (0.07–0.95 R_a) in the Baja California peninsula, indicating that the origin of helium is mainly crustal. Shear wave tomography demonstrates that the study area is associated with two low velocity regions, one in the crust and the other one in the upper mantle. Our results favor the hypothesis that the heating of fluids is produced by lower-crustal flow driven by strong topographic gradients along the rifted margin. This study provides new insights into the origin of the thermal anomalies in the LCB and the adjacent GC rift system.

Halite deposits have long been utilized for interrogating past climate conditions. Microthermometry on halite fluid inclusions has been used to determine ancient water temperatures. One notable obstacle in performing microthermometric measurements, however, is the lack of a vapor bubble in the single-phase liquid inclusions at room temperature. (Pseudo-) isochoric cooling of the inclusions to high negative pressures, far below the homogenization temperature, has commonly been needed to provoke spontaneous vapor bubble nucleation in the liquid. High internal tensile stress in soft host minerals like halite, however, may induce plastic deformation of the inclusion walls, resulting in a wide scatter of measured homogenization temperatures. Nucleation-assisted (NA) microthermometry, in contrast, employs single ultra-short laser pulses provided by a femtosecond laser to stimulate vapor bubble nucleation in metastable liquid inclusions slightly below the expected homogenization temperature. This technique allows for repeated vapor bubble nucleation in selected fluid inclusions without affecting the volumetric properties of the inclusions, and yields highly precise and accurate homogenization temperatures.

In this study, we apply, for the first time, NA microthermometry to fluid inclusions in halite and we evaluate the precision and accuracy of this thermometer utilizing (i) synthetic halite crystals precipitated under controlled laboratory conditions, (ii) modern natural halite that precipitated in the 1980s in the Dead Sea, and (iii) Late Pleistocene halite samples from a sediment core from Death Valley, CA. Our results demonstrate an unprecedented accuracy and precision of the method that provides a new opportunity to reconstruct reliable quantitative temperature records from evaporite archives.

Belemnites were a type of cephalopod abundant in the Jurassic and Cretaceous, whose fossils have been extensively used for paleo-oceanographic studies. For this study, we analyzed 69 elements by Inductively Coupled Plasma-Sector Field Mass Spectrometry (ICP-SFMS) in 15 belemnite rostra from the West Rodiles section in the Asturian Basin, Northern Spain. We aim to determine if belemnite rostra carbonate chemistry reflects changes in seawater chemistry during the Late Pliensbachian–Early Toarcian time interval and examine how belemnites can be used to trace known drivers of the Early Toarcian Oceanic Anoxic Event (T-OAE) and associated oceanographic processes.

Discarding drastic intra-species effects and assuming a similar influence of growth rates, diet, and other physiological processes for all analyzed belemnite specimens and taking into consideration the sizeable analytical uncertainty, we assess if determined belemnite rostra element chemistry reflects broad and relative changes in paleo-seawater chemistry. Many determined elements are present in only ppb amounts, and their interpretation is uncertain. We found that Mg, Mn, and P increase in the interval chronocorrelative to the T-OAE. This is interpreted to have resulted from an increase in these elements' inventory in seawater due to an increase in continental weathering and fluvial runoff associated with this global event. Iron, K, and Na contents decrease upwards in the section, potentially indicating that these elements became limited and likely hampered oceanic productivity. Our study also found a correspondence between a change in the behaviour of several elements, warming, the T-OAE negative CIE, and a reduction in the diversity and size of both calcareous nannofossils and belemnites in the study area.

Reconstructing the lithium isotope composition (δ⁷Li) of ancient seawater and lacustrine environment is crucial for tracing silicate weathering processes that regulate Earth's carbon cycle and climate. This study examines the reliability of evaporite minerals in preserving the Li isotope signature of the original brines from which they precipitate. We conducted evaporation experiments on modern natural waters from Qinghai Lake, the Yellow Sea, and Chaka Salt Lake, each with unique salinity and chemical composition, to examine the behavior of lithium isotope ratios in the evolving brine and precipitated evaporite minerals.

Our results show that initial hydrochemical conditions of the lake and ocean are the primary control on the evolution of major and trace elements and the mineral precipitation sequences during evaporative concentration. Aragonite, calcite, gypsum, and/or nesquehonite precipitated prior to halite. Li isotope fractionation of these non-halite precipitates varied significantly between 4‰ and 17‰. However, the δ⁷Li values of halite closely mirror those of the initial lake and seawater, indicating negligible Li isotope fractionation (<1‰) during halite precipitation. The δ⁷Li values of the evolving brines remain unchanged throughout the evaporation experiment. Solid phases precipitating before halite constitute <1–2% of the total moles of salt precipitated from the complete evaporation of the initial waters, thus, having little to no influence on the δ⁷Li values of evolving brines. Although halite comprises >77% of the total moles of salt, it does not fractionate Li isotopes. Therefore, halite more accurately reflects the δ⁷Li values of the co-evolving brine as well as those of the initial, unevaporated lake and seawater. These results underscore the potential of late stage evaporites, particularly halite, to serve as high-fidelity archives of ancient lacustrine and seawater δ⁷Li values. Such records are instrumental in reconstructing past silicate weathering and climatic conditions.

Speleothem paleoclimate records from the Peruvian Andes have been interpreted to reflect the strength of the South American monsoon. While these interpretations have been verified through comparison with other regional and global climate records, the mechanics of the cave environment that facilitate the preservation of this signal with such consistency remain unstudied. Here, we present four years of environmental data from Huagapo and Pacupahuain cave, and one year from Antipayarguna cave. The data reveal that the cave environment is very stable with little to no change in temperature and 100% relative humidity year-round. This stability in cave air is juxtaposed with the monsoonal drip water pulse that increases drip rates over 40 times on average across all seven monitored drip sites. Compared to the amount-weighted precipitation average δ¹⁸O_precip value, the cave drip water δ¹⁸O_DW values are evaporatively ¹⁸O enriched during infiltration through the soil/epikarst. As the monsoonal precipitation pulse fades and drip rates decrease, changes in the drip water chemistry (trace elements Mg/Ca and Sr/Ca, dissolved inorganic carbon δ¹³C_DW, and δ¹⁸O_DW values) indicate that prior calcite precipitation (PCP) drives the trace element and δ¹³C_DW variability. The δ¹³C_c and δ¹⁸O_c values of farmed slide calcite are highly variable. However, high drip rate and lower cave air pCO₂ during the monsoon combine to increase calcite precipitation rates. This causes speleothem records from these caves to be weighted toward annual monsoon conditions. Calcite isotope values from actively growing stalagmite tops support this finding. These results suggest that speleothems from these caves are sensitive to changes in monsoon precipitation amount, because it determines the duration of the monsoon drip water pulse, and therein, the extent of dry season PCP. Further, these data indicate that heterogeneity in the dolomitic limestone massif causes offsets between the carbon isotopes and trace metal concentrations between the caves, highlighting the need to normalize these datasets when chronology-stacking these proxies.

To investigate the role of carbonate recycling in generating ultrapotassic magmatic rocks, we carried out detailed MgZn isotope analyses of silica-saturated to silica-unsaturated ultrapotassic volcanic rocks from the Central Mediterranean basin. These rocks have lower Mg isotopic compositions (δ²⁶Mg = −0.40‰ to −0.21‰, n = 20) than the mantle and exhibit MORB-like Zn isotopic compositions (δ⁶⁶Zn = 0.23‰ to 0.30‰, n = 16). Postmagmatic alteration, crustal assimilation, magmatic differentiation, and diffusion processes were found to have insignificant effects on MgZn isotope variations of the ultrapotassic rocks. Based on the relationships between δ²⁶Mg and major and trace element contents and Nd isotopic compositions, the Mg isotope variations cannot be induced by adding carbonated eclogite to the mantle source. Instead, the low Fe/Mn, Hf/Hf^⁎, Ti/Ti^⁎ and high Ca/Al ratios and good correlations of Hf/Sm vs. Ca/Al and La/Yb vs. Ti/Eu suggest that the light Mg isotopic compositions of the ultrapotassic rocks were caused by carbonate melt metasomatism. In addition, the combined MgZn isotopic compositions suggest that the metasomatic agent is recycled carbonate-bearing silicate sediments. Silica-unsaturated ultrapotassic rocks have slightly lower δ²⁶Mg values than silica-saturated rocks, which is consistent with the greater amounts of carbonates in the recycled carbonate-bearing silicate sediments. The highly radiogenic Sr and unradiogenic Nd isotopic compositions of the studied rocks and two end-member mixing models further document that the mantle sources contained silicate-rich components, possibly Italian basements. The transition from silica-saturated to silica-unsaturated ultrapotassic rocks corresponds to the transformation of sediments from metapelites to carbonated metapelites. Therefore, by integrating Mg–Zn–Sr–Nd isotopic compositions, this study constrains the composition of subducted sediments and documents the important role of carbonate melt metasomatism in shifting the compositions of ultrapotassic rocks from silica-saturated to silica-unsaturated.